The retinal image of the human eye can deteriorate due to three causes: diffraction, aberrations and intraocular scattering. Diffraction is a property of the electromagnetic waves making up light and will consequently always be present in the retinal image. Its effect depends on the size of the pupil of the eye, being considerable only for small pupils (of the order of 2 mm or less), which virtually never occurs in normal vision conditions. The presence of aberrations and scattering in eyes of young subjects with normal visual conditions is low, but it increases considerably with age, the presence of ocular pathologies and refractive surgery interventions (I. IJspeert, J. K., de Waard, P. W., van der Berg, T. J., de Jong, P. T. (1990). The intraocular straylight function in 129 healthy volunteers: dependence on angle, age and pigmentation. Vision Research, 30(5), 699-707, Brunette, I., Bueno, J. M., Parent, M., Hamam, H., Simonet, P. (2003). Monochromatic aberrations as a function of age, from childhood to advanced age. Investigative Ophthalmology & Visual Science, 44, 5438-5446). Intraocular scattering in particular very significantly increases above normal values if ocular media transparency losses occur, such as those taking place in the crystalline with the development of cataracts.
The joint contribution of optical aberrations and intraocular scattering affects the quality of the retinal image. The double-pass technique (J. Santamaria, P. Artal, J. Bescos, “Determination of the point-spread function of human eyes using a hybrid optical-digital method”, J. Opt. Soc. Am. A, 4, 1109-1114 (1987)) based on projecting a collimated light beam in the retina of the patient, and directly recording the light reflected therein after the double-pass of the light through the ocular media allows obtaining the objective measurement of the contribution of aberrations and scattering to ocular optical quality (F. Díaz-Doutón, A. Benito, J. Pujol, M. Arjona, J. L. Güell, P. Artal, “Comparison of the retinal image quality obtained with a Hartmann-Shack sensor and a double-pass instrument”, Inv. Ophthal. Vis. Sci., 47, 1710-1716 (2006)).
Knowledge of the existence of ocular aberrations dates back to the middle of the 19th century. Low-order aberrations (defocus and astigmatism) can be measured using objective or subjective techniques and can be corrected using conventional lenses, contact lenses or refractive surgery interventions. Their impact on visual quality after their correction is therefore very low.
For measuring mid- and high-order (comatic, spherical . . . ) aberrations, different subjective and objective methods have been developed. There are currently several instruments based on these techniques which are used on a clinical level.
With respect to measuring intraocular scattering, there is no widely accepted robust method which allows the objective measurement thereof on a clinical level.
To date, most determinations of intraocular scattering have been performed using subjective methods of measurement. For example, the sensitivity to glare can be quantified by means of the equivalent background luminance (Stiles, W S. (1939) Discussion on disability glare at the 1939 CIE meeting in Scheveningen. Sekretariatsberichte der Zehnten Tagung CIE, 1942; Band I: 183-201, Vos, J. J. (2003). On the cause of disability glare and its dependence on glare angle, age and ocular pigmentation. Clinical and Experimental Optometry 2003; 86: 6: 363-370) technique based on the fact that the effect of the light scattered in the retina can be equaled by means of a background luminance and that it has led to the proposal of an equation for its quantification by the CIE (Comision Internacional de l′Eclaraige).
The direct compensation method is based on presenting an annular glare source with oscillating intensity and the compensation of its effect in the fovea through a central source of variable oscillating intensity in contrast with respect to that of the glare source. The straylight meter developed by Van der Berg is based on this method (Van den Berg, T. J. T. P. and Ijspeert, J. K. (1992). Clinical assessment of intraocular stray light. Applied Optics, 31, 3694-6).
The brightness visual acuity tester (Holladay, J. T., Prager, T., Trujillo, J., R. Ruiz. (1986). Brightness acuity test and outdoor visual acuity in cataract patients. Presented in part at the Symposium on Cataract, IOL and Refractive Surgery, Los Angeles.) is used to measure visual sensitivity and the power to discern between glare sources. It consists of an internally illuminated hemisphere with a hole in the middle. The patient holds the instrument close to his or her eye and observes a test through the hole. The latter provides a uniform glare source which can be used together with contrast sensitivity or visual acuity tests or charts.
Subjective methods have also been used which are combined with measurements of visual acuity and contrast sensitivity (J. Bailey, M. A. Bullimore (1991) A new test for the evaluation of disability glare. Optometry and Vision Science 68, 911-917). This type of measurement requires the active participation of the patient and can depend on many factors. Consequently, they are difficult and cumbersome to apply in clinical practice.
In addition, ophthalmologists normally use the slit lamp for routine cataract observation. A completely subjective method of classification and analysis has been developed using this observation (LOCS (Lens Opacities Classification System) III method (Chylack, L. T., Wolf J. F., Singer D. M., Leske, M. C., Bullimore, M. A., Bailey I. L., Friend, J., McCarthy, D., Wu, S. Y. (1993) The Lens Opacities Classification System III. Archives of Ophthalmology, 111, 831-836). Ophthalmologists must be specialized in this type of classification and results may differ among professionals. It must be taken into account that quantifying the grade of the cataract is of great interest for being able to determine the proper time to perform surgical intervention.
In recent years, methods have been developed to objectively determine intraocular scattering. However, most objective techniques and methods used are theoretical or experimental but not suitable for being adapted to a clinical setting, primarily due to the need to restrict variables affecting the measurement. In other words, the conditions in which the measurement is performed cannot be transferred to clinical practice as of today. It is possible to mention, for example, the dynamic light scattering measurement (Ansari, R. R., Datiles, M. (1999). Use of Dynamic Light Scattering and Scheimpflug Imaging for the Early Detection of Cataracts. Diabetes Technology and Therapeutics, 1(2), 159-168, Datiles, M., Ansari, R. R., Reed, G. F. (2002). A clinical study of the human lens with a dynamic light scattering device, Exp. Eye Res., 74, 93-102) or Scheimpflug imaging (Datiles, M., Magno, B., Friedlin, V. (1995). Study of nuclear cataract progression using the national Eye Institute Scheimpflug system, British Journal of Ophthalmology, 79, 527-534).
The double-pass (DP) technique by recording the image of a point on the retina contains information about aberrations and scattering. The contribution of the aberrations is located in the central part of the image, such that the more aberrated the eye is, the larger this central part. The effect of scattering is located basically in the outermost areas of the image such that the greater the scattering, the larger the peripheral image will be. Westheimer (Westheimer, G., Liang, J. (1994). Evaluating Diffusion of Light in the Eye by Objective Means. Investigative Ophtalmology and Visual Science, 35(5), 2652-2657, Westheimer, G., Liang, J. (1995). Influence of ocular light scatter on the eye's optical performance, J. Optical Society of America, 12(7), 1417-1424) combined subjective and objective measurements, verifying that intraocular scattering increases with age. Double-pass images were used in the objective part and a light scattering index was defined. However, the method developed was not robust to allow use on a clinical level. One of the main drawbacks of this technique is that the measurement is highly dependent on ocular aberrations. This is because the double-pass images are affected by both ocular aberrations and by intraocular scattering.
The use of a polarimetric technique by incorporating a polarimeter to a DP system to evaluate scattering has recently been suggested (Bueno, J. (2002). Polarimetry in the human eye using an imaging linear polariscope. Journal of Optics A: Pure and Applied Optics. 4, 563-561, Bueno, J., Berrio, E., Artal, P. (2003). Aberro-polariscope for the human eye, Optics Letters, 28(14), 1209-1211). This technique is based on the fact that light due to intraocular scattering is depolarized, whereas that forming the image in the retina maintains its polarization. Therefore, by evaluating the degree of depolarization of the light it is possible to evaluate the degree of intraocular scattering. However the low depolarization level of the retina for scattered light makes the application of the method unfeasible in clinical practice.
It must finally be pointed out that using the size of the spots of the Hartmann-Shack images has also been proposed for analyzing intraocular scattering (Applegate, R. A., Thibos, L. N. (2000) Localized measurement of scatter due to cataract. Investigative Ophthalmology and Visual Sciences (suppl), 41, S3).
All the systems and methods proposed for the objective evaluation of intraocular scattering are not robust, there still being no effective method for being used on a clinical level. In this context, it is undoubtedly advantageous to propose a new system and method for measuring light scattering in the eyeball or eye region, which can be readily adapted to clinical instrumentation. In fact, the new system and method presented allow objectively quantifying the degree of intraocular scattering and classifying the degree of development of a cataract, as well as objectively quantifying the degree of light scattering in the eye region caused by the tear film quality coating the cornea.